With over a million firefighters in the country servicing metro, rural communities, and wildlife forestry, the public depends on their optimal state of health. However, firefighters have twice as many cardiovascular deaths compared to police officers and four times the proportion among emergency medical responders. To date, despite the high cardiac risk for firefighters and its societal importance, there are no available physiological recordings during firefighting activities that may reveal why firefighters are dying on duty. Thus, the primary aim of the Surveying &Assessing Firefighter Fitness &Electrocardiograms: The SAFFE Study is to develop a non-invasive risk stratification approach to better identify firefighters at risk for cardiovascular events. For this exploratory/developmental study, 118 professional firefighters will be recruited to participate in the research protocol which includes (1) baseline ECG &V02 measures, (2) maximum capacity ECG &V02 during a graded exercise treadmill test and (3) on-duty ECG &VO2 measures. Continuous 12 lead ECG will be initiated at the beginning of the shift and maintained for 24 hours during all firefighter activities including: (1) fire calls, (2) medical calls, (3) training, (4) meals, (5) exercise, and (6) rest/sleep. Intermittent V02 measures will be obtained throughout the shift during select periods including: fire calls, medical calls and training. The proportion of firefighters that demonstrate specific ECG abnormalities and nonlinear HR/V02 relationship will be reported at baseline, maximum capacity and on-duty. With cardiac events as one of the leading causes of death, the use of biological measures (ECGs &V02) to better identify at-risk first responders who service the public, would aid in assessing risk and evaluating intervention strategies that optimize health. PUBLIC HEALTH RELEVANCE With over a million firefighters in the country servicing metro, rural communities and wildlife forestry, the public depends on their state of health. However, firefighters have twice as many cardiovascular deaths compared to police officers and four times the proportion among emergency medical responders. With cardiac events as one of the leading causes of death, the use of biological measures (ECGs &V02) to better identify at-risk first responders who service the public, would aid in assessing risk and evaluating intervention strategies that optimize health.